Embolism
An experimental model of pulmonary embolism was used by the inventors to study the utility of different compositions with regard to the delivery of gaseous NO. Therefore, and with the aim to facilitate the understanding of the invention, background information about embolism is given below. The invention is however applicable to the prevention, alleviation or treatment of any condition, where the delivery of nitric oxide is beneficial, for example through its vasodilatory effects. The below background is thus given for illustrative purposes only, and it is not to be interpreted as limiting the scope of the invention.
An embolus is a foreign object, a quantity of air or gas, a fat globule, a bit of tissue or tumour, or a piece of a thrombus that circulates in the bloodstream until it becomes lodged in a vessel, partially or completely obstructing blood flow.
Pulmonary embolism is a common disorder accompanied by a significant morbidity and mortality. Thromboembolism may either be acute through activation of the blood clotting system and disseminated intravascular coagulation, or occur at a later stage through the formation of thrombi in the pulmonary vessels or formation in the venous circulation with subsequent embolization to the lung. The cause behind thrombi in the lung can also be so called thrombotisation, i.e. the formation of microthrombi in the circulation, triggered by tissue factors in the blood vessels. These microthrombi travel in the circulation until becoming trapped in the capillaries in the lung. It is estimated that up to 40% of all cases of pulmonary embolism may be of this origin.
It is also estimated that pulmonary embolism is the main or at least a contributory cause of in-patient death. Swedish autopsy records indicate that pulmonary embolism is involved in about 20% of in-patient deaths. Pregnant women, and in particular women undergoing caesarian section; cancer patients; trauma victims, and patients undergoing surgery, e.g. orthopedic surgery, are at risk. Further risk groups include, but are not limited to, individuals confined to bed rest or other types of confinement or restriction in the movement of the body or limbs, both during medical treatment or recovery from such treatment, or during transportation, e.g. air travel. Still further risk groups include, but are not limited to patients with infections, suffering from diseases or undergoing pharmaceutical treatments disturbing the blood clotting system or the system for resolution of blood clots.
One special form of pulmonary embolism, pulmonary gas embolism, is a well-known consequence of surgery, trauma, diving and aviation, including the exploration of space. Another form, pulmonary thromboembolism, is caused when a thrombus or fat globule travels in the blood to the lungs as a result of trauma, surgery or dislodging of a thrombus or part thereof from another location in the body, e.g. in deep venous thrombosis (DVT).
In the majority of the cases of pulmonary embolism, the source is deep venous thrombosis (DVT). Venous thromboembolic disease is the third most common cardiovascular disease after ischemic coronary heart disease and stroke. The most common treatments of pulmonary embolism include the administration of nasal oxygen, infusion of anticoagulantia and/or trombolytica, and surgical intervention. In the administration of thrombolytica, bleeding is a serious complication, which has to be considered. Inhaled nitric oxide (NO) has been tried experimentally in pulmonary embolism, but consensus has not been reached if such treatment is efficacious or not.
The pathophysiology of pulmonary embolism is pulmonary macro- or micro-obstruction, depending on emboli size, leading to pulmonary hypertension of varying severity. Acute pulmonary hypertension may cause right ventricle failure (acute cor pulmonale) and eventually cardiogenic chock. Treatment of acute pulmonary hypertension must therefore include reduction of pulmonary afterload, preferably pulmonary vasodilators with no systemic effects. Another feature of pulmonary embolism is disturbances of blood gases of varying degree, indicating ventilation-perfusion matching failure, though normal or disturbed blood gases are not conclusive for pulmonary embolism. For example, PaO2 is likely to be decreased after acute massive pulmonary embolism but may be normal in patients with sub-massive pulmonary embolism.
For more information on pulmonary embolism, see “Guidelines on diagnosis and management of acute pulmonary embolism”, Task Force on Pulmonary Embolism, European Society of Cardiology, European Heart Journal 2000; 21:1301-1336.
Insufficient perfusion also occurs in other instances, such as transplantation and in individuals confined to bed rest or other types of confinement or restriction in the movement of the body or limbs, both during medical treatment or recovery from such treatment, or during transportation, e.g. air travel.
Nitric Oxide
Nitric oxide (NO) is a molecule of importance in several biological systems, and is continuously produced in the lung and can be measured in ppb (parts per billion) in expired gas. The discovery of endogenous NO in exhaled air, and its use as a diagnostic marker of inflammation dates back to the early 1990-ies (See e.g. WO 93/05709; WO 95/02181). Today, the significance of endogenous NO is widely recognized, and since a few years back, a clinical analyzer is available on the market (NIOX®, the first tailor-made NO analyzer for routine clinical use with asthma patients, AEROCRINE AB, Solna, Sweden).
In the summer of 1997 the European Respiratory Journal published guidelines (ERS Task Force Report 10:1683-1693) for the standardization of NO measurements in order to allow their rapid introduction into clinical practice. Also the American Thoracic Society (ATS) has published guidelines for clinical NO measurements (American Thoracic Society, Medical Section of the American Lung Association Recommendations for standardized procedures for the online and offline measurement of exhaled lower respiratory nitric oxide and nasal nitric oxide in adults and children—1999, in Am J Respir Crit. Care Med, 1999; 160:2104-21 17).
In early experiments attempting to elucidate the role of NO in respiratory gas, massive helium or air emboli were used to stop the circulation in the lungs of test animals. The results indicated that increased levels of NO could be detected in exhaled air (Gustafsson et al., Endogenous nitric oxide is present in the exhaled air of rabbits, guinea pigs and humans. Biochem Biophys Res Commun 1991; 181:852-7).
In 1999, Deem et al. published results indicating that haemodilution during venous gas embolization improves gas exchange, without altering V(A)/Q or pulmonary blood flow distributions (Anesthesiology., 1999 December; 91(6):1861-72). A continuous infusion of nitrogen through the left internal jugular vein at a rate of approximately 0.006 ml/kg min was applied to achieve embolization. In the results, an increase of NO is recorded in embolised, haemodiluted anemic test animals, but not in undiluted controls. The authors state that the difference between baseline and T 1 VNO was statistically significant only for anemic animals. In practice, no increase in NO was recorded for animals with normal hematocrit. Deem et al. also discuss the limitations of the model, venous gas embolization using a continuous infusion of small bubbles, and states that it may be dissimilar to cases of air embolization in the clinical setting, and that extrapolation of the data to clinical management is difficult.
Thθ effect of vasodilator therapy was investigated in a canine model of acute pulmonary hypertension (Priebe, Am. J. Physiol. 255 (Heart Circ. Physiol, 24):H1232-H1239, 1998). In this study, pulmonary embolization was simulated by injecting a suspension of finely chopped muscle tissue in saline containing 2000 U of heparin. Small volumes (0.5-2 ml) of the muscle suspension was injected repeatedly through a femoral vein catheter until the mean pulmonary arterial pressure had increased approximately threefold.
It is generally recognised that endogenous generation of the gaseous molecule nitric oxide (NO) plays an important role in the modulation of pulmonary vascular tone to optimise ventilation-perfusion matching (Persson et al. 1990). In healthy human adults, NO is of importance in regulation of both basal pulmonary and systemic vascular resistance (Stamler et al. 1994). Local regulation of blood flow is influenced by administration of NO synthase inhibitor in healthy human subjects (Rimeika et al 2004). Vasodilator effects of endogenous NO in the postnatal pulmonary circulation clearly contribute to the adaptations of the fetal lung to air breathing at delivery (Abman et al. 1990). NO generation in the postnatal lung is stimulated for example by mechanical stretch, increased shear forces and increased O2 tension in the alveoli (Heymann 1999). Measuring NO in exhaled breath is a good way of monitoring changes in endogenous NO production or scavenging in the lung (Gustafsson et al. 1991).
Since ventilation-perfusion matching disturbances and increased pulmonary artery blood pressure are features of pulmonary embolism, inhaled NO has been tested as treatment. Nevertheless treating pulmonary embolism with inhaled NO has yielded conflicting results e.g. improvement of hemodynamics but no improvement of blood gases (Tanus-Santos J E & Theodorakis M J, 2002).
Further, U.S. Pat. No. 5,670,177 discloses a method for treating or preventing ischemia comprising administering to a patient by an intravascular route a gaseous mixture comprising NO and carbon dioxide CO2 wherein the NO is present in an amount effective to treat or prevent ischemia.
U.S. Pat. No. 6,103,769 discloses a similar method, with the difference that saline, saturated with NO, is used.
The published international application WO 94/16740 teaches the use of NO delivering compounds, such as S-nitrosothiols, thionitrites, thionitrates, sydnonimines, furoxans, organic nitrates, nitroprusside, nitroglycerin, iron-nitrosyl compounds, etc, for the treatment or prevention of alcoholic liver injury.
Nitrates are presently used to treat the symptoms of angina (chest pain). Nitrates work by relaxing blood vessels and increasing the supply of blood and oxygen to the heart while reducing its workload. Examples of presently available nitrate drugs include:
Nitroglycerin (glyceryl nitrate) (1,2,3-propanetriol-nitrate), which is today mostly taken sublingually to curb an acute attack of angina. Strong headaches and dizziness due to the rapid and general vasodilatory effect are frequently encountered side-effects. Nitroglycerin infusion concentrates are also available, and diluted in isotonic glucose or physiological saline for intravenous infusion.
Isosorbide mononitrate (1,4:3,6-dianhydo-D-glucitol-5-nitrate), which is taken as prophylactic against angina pectoris. Tolerance development is a problem in long-term treatment regimens. Frequent side-effects include headache and dizziness, as encountered with nitroglycerin.
Isosorbide dinitrate (1,4:3,6-dianhydo-D-glucitol-2,5-nitrate), which is taken both acutely and prophylactically against angina pectoris and cardiac insufficiency.
Pentaerythrityl nitrates, a group of organic nitrate, are known to exert long-term antioxidant and anti-atherogenic effects by as yet unidentified mechanisms. Pentaerythrityl tetranitrate has been investigated in the context of nitrate tolerance, an unwanted development in nitrate therapy, and experimentally tested in pulmonary hypertension.
Inorganic nitrates, such as potassium nitrate and sodium nitrate, have a long use as food preservatives. Nitrate has in general been considered to be potentially harmful, due to the theoretically possible formation of carcinogenic N-nitroso compounds in food, and in humans in vivo. Lately, the role of dietary nitrate has been reevaluated, in particular as the endogenous production of NO in the arginine-nitric oxide system and its role in host defense has been discovered.
L-arginine, and esters thereof, such as the ethyl-, methyl- and butyl-L-arginine have been used to increase the endogenous production of NO.
Among the compounds and compositions presently available, many are associated with undesired properties or side-effects, such as toxicity problems, stability problems, delayed action, irreversible action or prolonged action, etc.
One objective behind the present invention was to identify new compositions and method for the delivery of NO, or for providing a source of NO, in the treatment, alleviation and/or prevention of conditions, where the administration of NO is believed to be beneficial. One illustrative example of such conditions is insufficient perfusion, as exemplified by particular pulmonary embolism.
Another objective was to identify a method and composition, which makes it possible to administer NO in a safe and efficient fashion, and which does not exhibit the side effects or tolerance development associated with conventional treatments and drugs.
Other objectives, the solutions reached and the advantages associated therewith will become evident upon study of the description and examples.